A liquid crystal display apparatus includes a liquid crystal display panel which has a display surface on a front side, and a backlight which illuminates the liquid crystal display panel from a back side of the liquid crystal display panel.
Hereinafter, an example in which the liquid crystal display panel is disposed in a vertical position will be described.
Conventionally, an edge light type backlight has been proposed. The edge light type backlight includes a rectangular light guide plate disposed in a vertical position on the back side of the liquid crystal display panel, and a light source unit disposed to face a lower end face of the light guide plate.
Light emitted by the light source unit is transmitted through the lower end face of the light guide plate, and made incident inside of the light guide plate, and further emitted to the front side of the light guide plate from a front surface of the light guide plate. As a result, the light guide plate performs a surface emitting.
By the surface emitting of the light guide plate, the liquid crystal display panel is illuminated from the back side. When the surface emitting of the light guide plate is uniform, the display surface of the liquid crystal display panel is uniformly lightened.
In order to guide the light incident inside of the light guide plate to the front side of the light guide plate, a reflection sheet is disposed on the back side of the light guide plate through an air layer. In addition, a plurality of light scattering dots are formed on a rear surface of the light guide plate. An area of each light scattering dot disposed near the light source unit is small, and the area of each light scattering dot disposed away from the light source unit is large.
The light transmitted through the rear surface of the light guide plate and made incident to the light scattering dot is scattered to the front side.
The light transmitted through the rear surface of the light guide plate and made incident to the air layer is reflected to the front side. The reason is that, since the refractive index of the air layer is lower than the refractive index of the light guide plate, a total reflection to the front side may occur at a boundary portion between the light guide plate and the air layer. In addition, the reason is also that the light transmitted through the air layer is reflected to the front side by the reflection sheet.
Preferably, the reflection sheet is adhered to the light guide plate. The reason is that, when the reflection sheet is not adhered to the light guide plate, the reflection sheet may be deflected due to a thermal expansion, for example.
If the reflection sheet is deflected, the surface emitting of the light guide plate becomes uneven (so-called brightness unevenness is generated). As a result, display quality is deteriorated.
In addition, a surface light emitting device including a plurality of light scattering dots with adhesiveness formed on the rear surface of the light guide plate has been proposed (see Japanese Patent Application Laid-Open No. 2013-93195).
In the case of a first surface light emitting device described in Japanese Patent Application Laid-Open No. 2013-93195, the reflection sheet is adhered to the light guide plate through the light scattering dots. An air layer is present between the light scattering dots.
In the case of a second surface light emitting device described in Japanese Patent Application Laid-Open No. 2013-93195, the light scattering dots and the rear surface of the light guide plate exposed between the light scattering dots are covered with a low refractive index layer. The rear surface of the low refractive index layer is covered with an adhesive layer. The reflection sheet is adhered to the light guide plate through the adhesive layer.
All the first and second surface light emitting devices described in Japanese Patent Application Laid-Open No. 2013-93195 satisfy both of that the reflection sheet is adhered to the light guide plate and that a layer having a lower light refractive index than the light guide plate comes into close contact to the rear surface of the light guide plate.
If a layer having the light refractive index higher than the light refractive index of the light guide plate comes into close contact to the rear surface of the light guide plate, it would be difficult for the light to be totally reflected at the boundary portion between the layer and the light guide plate. Therefore, light-guiding efficiency to the front side of the light guide plate is deteriorated.